1. Field of the Invention
The present invention relates to a display device, and more particularly to a liquid crystal display panel and a fabricating method thereof. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for preventing waviness in the liquid crystal display panel and maintaining a cell gap of the liquid crystal display panel.
2. Description of the Related Art
Generally, a liquid crystal display device LCD controls the light transmittance of liquid crystal cells in accordance with a video signal, thereby displaying a picture on liquid crystal display panel that corresponds to the video signal. More particularly, the liquid crystal display device includes a liquid crystal display panel in which liquid crystal cells are arranged in a matrix. Drive circuits for driving the liquid crystal cells are also positioned on the liquid crystal display panel.
FIG. 1 is a cross-sectional diagram of a related art liquid crystal display panel. The liquid crystal display panel shown in FIG. 1 includes an upper array substrate 2, which is also known as a color filter array substrate 2, and a lower array substrate 22, which is also known as a thin film transistor array substrate. A liquid crystal area 5 is in a cell gap between the upper array substrate 2 and the lower array substrate 22. The cell gap of the liquid crystal area 5 is maintained by spacers 13 positioned between the upper array substrate 2 and the lower array substrate 22.
A first thin film pattern 4, such as a black matrix and a color filter is formed on the upper array substrate 2. A second thin film pattern 24, such as a thin film transistor, a signal line and a pixel electrode, is formed on the lower array substrate 22. The spacers 13 are formed on the upper array substrate 2 to define the liquid crystal area 5 by keeping a cell gap between the upper array substrate 2 and the lower array substrate 22.
In such a related art liquid crystal display panel, the upper and lower array substrates 2 and 22 are provided and then a liquid crystal dropping process for spreading the liquid crystal over at least one of the upper array substrate 2 and the lower array substrate 22 is performed. Subsequently, the upper array substrate 2 and the lower array substrate 22 are bonded together with both spacers 13 and the liquid crystal area 5 between the upper array substrate 2 and the lower array substrate 22.
Right after the related art VALC process is performed, the liquid crystal is concentrated at a center part of the liquid crystal display panel. Thus, the cell gap of the center part of the liquid crystal display panel is thicker than the rest of the liquid crystal display panel right after the bonding process. After awhile, the liquid crystal gradually spreads out such that cell gap of the center part of the liquid crystal display panel decreases while the cell gap of the rest of the liquid crystal display panel increases. The cell gap will eventually become uniform across the entire liquid crystal display panel in accordance with the height of the spacers 13.
Friction occurs between the spacer 13 and an area of the lower array substrate 22 that is in contact with the spacer 13. This friction can generate a problem in that internal structures of the liquid crystal display panel can be damaged, such as the signal lines. FIG. 2 is a cross-sectional diagram of a related art liquid crystal display panel having a spacer in contact with a protrusion 25 on the lower array substrate. To prevent such internal structure damage, the protrusion 25 is formed on the lower array substrate 22 and contacts the spacer 13, as shown in FIG. 2. Accordingly, the problem of a frictional force between the spacer 13 and the lower array substrate 22 causing damage is solved.
As shown in the dotted circle magnified portion of FIG. 2, the pressure is concentrated at a center part of the spacer 13 that is in contact with the protrusion 25. This pressure prevents the spacer 13 from moving along the lower array substrate 22. However, this pressure can cause the problem of not being able to alleviate waviness in the upper array substrate 2 because of the large static friction between the spacer and the protrusion. The waviness degrades the uniformity of the cell gap across the liquid crystal display panel.
FIG. 3 is a diagram of the waviness in an upper array substrate. The upper array substrate 2 can have a waviness with a cycle of 5˜20 mm and size of 0.02˜0.03 μm before the bonding process. Right after the upper array substrate 2 is bonded to the lower array substrate 22, a force FEXT applied from the outside to the inside of the liquid crystal display panel is greater than a force FINNER applied from the inside of the liquid crystal display panel to the outside of the liquid crystal display panel. The mathematical relationship of the forces are as follows.FEXT>FINNER,(FINNER=FLC+FCS)  [MATHEMATICAL FORMULA 1]whereFEXT=a force applied from the outside to the inside of the panel,FINNER=a force applied from the inside to the outside of the panel, which is opposite to the force from the outside,FLC=a force with which the liquid crystal pushes the substrate, andFCS=a force with which the protrusion pushes the spacer.The waviness spreads across the entire liquid crystal display panel after some time elapses such that the force FEXT applied from the outside to the inside of the liquid crystal display panel becomes equal to the force FINNER applied from the inside to the outside of the liquid crystal display panel.
An upper array substrate 2 with such waviness can be leveled when the force FEXT applied from the outside to the inside of the liquid crystal display panel is equal to the force FINNER applied from the inside to the outside of the liquid crystal display panel, as shown in MATHEMATICAL FORMULA 2 below.FEXT=FINNER,(FINNER=FLC+FCS),(FCS=PCS*ACS)  [MATHEMATICAL FORMULA 2]wherePCS=a pressure applied to a contacting area between the spacer and the protrusion, andACS=an area of the contact area between the spacer and the protrusion.However, the spacers 13 contacting the protrusions 25 can only be partially depressed and are held in place on the protrusions, which results in a contact density between the spacers 13 and the protrusions 25 of about 50 ppm and a recovery rate of the spacer 13 is about 60% or lower. FIG. 4 is an experimental data showing the non-uniformity of a cell gap in the related art liquid crystal display panel. Because the protrusions 25 can only be partially depressed causing a static friction between some of the spacers and the protrusions, pressure is transmitted up to the entire upper array substrate 2 through some of the spacers 13 such that waviness of the upper array substrate 2 can not leveled and the cell gap is uneven as illustrated in an experimental result shown in FIG. 4. The non-uniformity of the cell gap causes imperfections in the images on the liquid crystal display panel.